Sorry I missed out the "how do you explain the foriegn run plants" bit. Foriegn car plants where only really viable once the union laws were changed to make it more attractive to the Japs et al. They also tended to agree on single union agreements too. This of course upset the indiginous Marxist union leaders and in one notable example, I think it was Toyota, in Dundee, they went and built the factory somewhere in Europe in the end, could have been 2000+ jobs for Dundee, still, we told them capitalist barstewards eh brothers. Still by then our own car industry had already pretty much perished otherwise it may have been able to benefit from the new union laws.
Of course there is/was a lot of bad management, no question, but is doesn't matter how good/bad your management is, the Unions formed a major obstacle that first needed removal.